The overarching objective of this research is to develop new synthetic organic methodology that will provide efficient ways to prepare structurally-complex biomedically-relevant molecular scaffolds from simple starting materials. Polycyclic nitrogen containing heterocycles and medium or large ring systems are structural motifs that are ubiquitous in medicines and biologically active compounds. However, these structural motifs can be challenging to prepare, which can limit their use in biomedical studies. New methods to prepare these motifs would benefit biomedical research by providing synthetic routes to important compounds. Our proposed research is based on our recent discovery of a novel ring fragmentation that provides tethered aldehyde ynone products in high yield. Our central hypothesis is that this ring fragmentation will provide us with a unique and efficient way to prepare both polycyclic nitrogen containing heterocycles and medium or large ring systems. These synthetic methods will be directly applicable to the synthesis of biologically active natural products and we propose to prepare several natural products over the course of this grant period. The specific aims for this proposal are to study the fragmentation of fused bicyclic ?-silyloxy-?-hydroxy-?-diazo carbonyl compounds in which the ring fusion bond breaks as a way to prepare medium and large sized cyclic ynones and cyclic ynoates. To apply the ring fragmentation of fused bicyclic ?-diazo esters to the synthesis of (-)-phoracantholide I, (-)-diplodialide C, and the kinase inhibitor resorcylic macrolide L-783,277. To further develop the use of tethered aldehyde ynoate fragmentation products as substrates for intramolecular 1,3-dipolar cycloaddition reactions in order to prepare polycyclic 2,5-dihydropyrroles. To apply the ring fragmentation / 1,3-dipolar cycloaddition sequence to concise syntheses of the aeruginosin alkaloid core (choi), (-)-mesembrine, demissidine and (+)-aspidospermine.